Labour Party members will be the ultimate judge on whether MPs disloyal to Jeremy Corbyn should be deselected, one of his leading union backers has said.

Matt Wrack, the General Secretary of the Fire Brigades Union, told the HuffPost UK the attacks on Corbyn’s leadership from Labour MPs had already damaged the party’s prospects of winning the 2020 General Election.

Wrack, who rejoined Labour this year, said Corbyn was a break with the “consensus” of the past 30 years, and he could be swept to power on wave of support from millions of young people eligible to vote for the first time in 2020.

The trade unionist is one of Corbyn’s most vocal backers, and has spoken at Momentum rallies in support of the leader and his policies.

For the second summer in a row Labour is engulfed in a leadership contest, and Wrack said the fact Corbyn’s rival candidate is positioning himself as anti-austerity shows how much the debate has changed in the past 12 months.

Asked by the HuffPost UK whether MPs who refused to accept a Corbyn victory should face deselection, Wrack said: “I hope a number of people will accept that [a Corbyn victory] and move on.

“The question of reselection isn’t for me to say, it’s a matter for local members to judge that.”

Wrack also spoke out against abuse some pro-Corbyn MPs had received, and claimed “there is bullying going in the Parliamentary Labour Party.”

His comments came less than a day after Lancaster and Fleetwood MP Cat Smith, the Shadow Minister for Voter Engagement who nominated Corbyn last summer, revealed a colleague swore at her in Parliament.

Glad to be home for summer. A colleague swore at me today. I hope recess gives space to reflect how we treat those around us on Team Labour.

After the mass resignation of Labour MPs from the Shadow Government in the wake of the EU referendum, Wrack joined thousands of pro-Corbyn supporters in a Parliament Square.

Wrack told HuffPost UK he had enormous respect for those who had refused to resign, such as Andy Burnham and Emily Thornberry, but was angry at the MPs who had walked out.

He said: “I think the biggest thing that’s caused harm to Labour’s electability is the behaviour of Labour MPs after the referendum.

“One thing that will do significant damage is the determined effort to get rid of the leader.”

As far as Wrack is concerned, Corbyn can steer Labour into Government by reconnecting with working-class communities across the country which he believes were ignored by Tony Blair.

“Blair felt we don’t need to listen to Labour voters as they will always vote Labour,” he said, before adding: “Blair during that period lost millions of Labour voters.”

When asked whether there were any lessons that could be taken from New Labour in the run up to the 1997 election victory, Wrack replied: “There’s a question whether Blairism won that or whether opposition to the Tories meant Labour was going to win. “

Despite claims from Corbyn’s opponents that he will be unable to reach out beyond Labour heartland into Tory seats at the next election, Wrack believes the party leader will be able to win over voters in southern England.

He said: “I think that can be done. Many of those seats seen as Tory seats or Tory swing seats are actually predominately working class constituencies – the Basildons of this world. Labour needs to get in there and explain their programme for Government.”

Wrack is enthused by the political awakening sweeping across certain parts of the country, and said he is unable to remember a time in the UK went rallies were so well attended and people took to the streets in such numbers to support a leader.